Set fish aside. Bring next 9 ingredients to a boil in a large skillet. Reduce heat, cover and simmer for 15 minutes. Arrange two of the steaks in the broth. Cover and simmer for about 10 minutes, turning once, until the fish flakes to the touch of a fork. Transfer to a greased shallow baking dish an repeat with the two remaining steaks. Boil the broth, uncover, until reduced to 1 c. Strain through severa thcknesses of cheesecloth. In a saucepan melt 1 tbsp. of butter. Add the flour and stir until blended. Bring the milk to a boil and then add all at once to the butter/flour mixture, stirring vigorously until the sauce is thickened and smooth. Stir in the strained broth and cheeses. Beat the egg yolk with a little of the warm sauce before adding to the saucepan to prevent curdling. Continue to cook the sauce over low heat, stirring until thick. Stir in the remaining butter and the cream. Season with salt and pepper. Pour the sauce over the fish and boil until lightly brown.